Zinc oxide



H. M. CYR

ZINC OXIDE AMay 14, 1940.

Filed Nov. 27, 1955 2 sheets-sheet 1 INVENTOR HOWARD M CYR BY @WLM/M* ATTORN EYS Patented May 104, 1940 UNITED STATES TENT 'or-FICE ZINC OXIDE Application November 27, 1935, Serial No. 51,748 l Claims.

This invention relates to the manufacture of zinc oxide, and has for its object the provision of certain improvements in the method of manufacturing zinc oxide by the oxidation of zinc 5 vapor in consequence of which improvements the zinc oxide particles are well developed and relatively coarse crystals, i. e., particles showing a distinct crystal form under the microscope. More particularly, the invention contemplates an im- '10 proved method of producing zinc oxide consisting substantially of crystals of acicular habit. Such acicular crystals may be obtained in the form of long thin needles or in the'form of twins, threelings, or fourlings. The method of the invention also contemplates the production of zinc oxide consisting of columnar or spear-head shaped or platy crystal forms.

l Acicular crystals are occasionally observed in certain types of commercial zinc oxide, particularly those made by the American'process. However, in the customary methods of making zinc oxide any occurrence of acicular crystals is adventitious, and the usual commercial grades of zinc oxide are substantially free of crystals of acicular habit. The present invention aims to provide a method of producing'zinc oxide in which the form and size of the zinc oxide par-V ticles are so controlled as to obtain at will zinc oxides of any desired type of relatively coarse well-developed crystals, thereby making possible the production of zinc oxide consisting substanindividual crystals of zinc oxide have opportunityl to grow freely. The zinc vapor to be oxidized by 40 the method of the invention may be evolved either by volatilizing metallic zinc in any suitable vtype of retort or by reducing zinc ore with fuel in a zinc retort of any appropriate type or in a zinc oxide furnace of the Wetherill (fiat grate) or 45 traveling grate type. The zinc vapor (evolved by any of these means) is diluted with a non-oxidizing gas, preferably a gas of reducing nature such as producer gas or gas evolved by the destructive distillation of coal rich in volatiles vsuch as 50 bituminous coal. The mixture of zinc vapor and diluting gas is then burned in a slow flame in a combustion chamber maintained at a controlled temperature above 950 C. and preferablyabove 1000 C. and less than 1200 C., although under 55. certain conditions acicular zinc oxide may be obtained at temperatures exceeding 1200 C'. The zinc oxide being formed by the combustion of zinc vapor is retained in the combustion chamber and exposed to the controlled high temperature prevailing there for a suflicient period to permit 5 the crystals to grow to the desired degree. To bring about this retention of the zinc oxide in the zone of high temperature, it is advantageous to construct the combustion zone in which the zinc vapor is burned with a volume relatively k10 large in proportion to the volume of Zinc vapor i entering it. For example, the combusion chamber may advantageously be of suicient lvolume to hold 25 to 50 times the volume of zinc vapor that enters the chamber per minute. l5

No substantial oxidation ofthe zinc vapor should occur until it reaches the combustion chamber` Where the source of zinc vapor is a volatilizaton retort (for metallic zinc) or a reduction retort (for zinc ore) only ordinary care 20 is required to prevent premature oxidation of zinc vapor. AWhere the source of zinc vapor is a zinc oxide furnace of the Wetherill grate or traveling grate type in which air is blown through a charge of zinc ore and reducing material sup- 25,

ported on a perforated grate, care should be taken to operate the zinc oxide furnace in such manner as not to oxidize substantial amounts of the zinc vapor evolved therein before the zinc vapor has reached lthe high temperature combustion 30 chamber. Moreover, care should be taken not to y dilute the zinc vapors with excessive amounts of atmospheric nitrogen, in order to avoid too rapid gas flow into the combustion chamber where the zinc Vapor is burned to zinc oxide, which might y interfere with the orderly growth of zinc oxide crystals at the optimum rate, and thus interfere with the `formation of well-developed acicular crystals. Accordingly, it is desirable to restrict the amount of air blown throughthe grate of the zinc oxide furnace in order to avoid the passage of excess air through the charge, which would result in the oxidation of zinc vapor to zinc oxide immediately above the charge, as well as in diluting the zinc vapor from the furnace with excessive amounts of nitrogen. It is likewise desirable to maintain a sufciently high temperature in the zinc oxide furnace in order to avoid oxidation of zinc vapor to zinc oxide` by carbon dioxide produced by the oxidation of the reduction fuel in the charge. Ai temperature of 1200 C. will generally be adequate for this purpose, but higher temperatures may be maintained if desired. It may also, under certain circumstances, be 4desirable to lower the content of oxi- 55 above 950 C. and preferably about 1000 C. may be brough-t about by supplying air tothe combustion chamber in amounts only slightly in excess of that required to'oxijdize' the zinc vapor and any reducing gases present, and at a slow rate, whereby the zinc vapors become diluted with atmospheric nitrogen. The zinc vapor may lil ewise be advantageously diluted with reducing gases such asproducer gas, illuminating gas and the equivalents thereof. The ratio by volume of the diluting gas, vsuch as producer gas, to the zinc vapor may be varied for example, between 2 to 1 and l5 to l, although' vthese figures are given merely by way of example, since under certain circumstances more or less producer gas or other l diluting gas may be used. If, however, the dilutngvgas is substantially less involume than twice the volumeof the zincvapor to be oxidized, the acicularrzinc oxide formed may tend to collect on the walls of the combustion l chamber in which it is formed`(particularly when the diluting gas is atmospheric nitrogen), and may not be carried out in its entirety to the system for collecting the zinc oxide. This difliculty may be overcome by periodically injecting -jets of air or -gasinto the combustion chamber in such manner as to .blow the zinc oxide from the walls of the combustion chamber.

Clogging of the orifices through which the zinc vapor-,bearing gases or air are introduced into the combustion chamber by rock oxide forming on such orifices may be obviatedby surrounding the jets of zinc vapor or air with annular jets of nitrogen or reducing gasso that combustion of zincvapor to zinc oxide does not occur in the immediate neighborhood of the orifices.

, The velocity of gas movementlin the combustion chamber should be adequate to bring about thorough -mixing of the combustion air, zinc vapor and diluting gas. gas movement should moreover be such as not to remove the particles of zinc oxide from the combustion chamber before they have had an opportunity to grow to the desired acicular form and likewise not to remove unconsumed zinc vapor from the combustion chamber. The combustion chamber may be provided with baies or the like to produce turbulent or tortuous gas ow and thus facilitate intimate mixing of the gases.

'I'he diluting gas may be mixedwith the zinc vapor either beforeor after it enters the combustion chamber in which it-.is burned tozinc oxide. Reducing gas may advantageously be mixed with the zinc :vapor before it enters the combustion chamber in order to neutralize the oxidizing effect of any air that mayleak into the system. in advance of thecombustion chamber and thus to prevent premature oxidation of zinc vapor outside the Lcombustion chamber. Slow combustion of the zinc vapor to zinc oxide may be brought about by retarding oxidation by dilution with the diluting gas and by slow and restricted supply of oxidizing gas'such as air. Y If the dilutinggasf used is a reducing gas such as producer gas, the

It may The velocity and character of reducing gas will retard the oxidation by combining with some of the oxidizing gas supplied. Reducing gas used as a diluent has three'advantageous effects in the combustion chamber, namely: (1) it acts as a diluent for Athe zinc vapor, (2') it combines WithsOme of the available oxidizinggases and thus decreases the rate of oxidation of the zinc vapor, and (3) it racts as aV carrying agent for the oxide formed, and thus facilitates conveying it to the collecting system.

VThe desired controlled high temperature in the combustion chamber may be produced by supplying regulated amounts of external heat to the combustion chamber, by conserving the heat produced by the combustion of the zinc vapor and producer gas in the chamber, as for example, by

kheat insulating the combustion chamber Vor,

purposes, such for example, as the reheating flues in the so-called WesternI type of zinc oxide furnace. j

It is my present preferred practice to use producer gas as a diluent for the zinc vapor. Gases evolved by the destructive distillation of coal, such for example as illuminating gas, may be used with advantage as the diluent for the' zinc vapor, as hereinbefore' pointed out. Where the source of zinc vapor is a zincoxide furnace of the Wetln erill grate or travelling gra-te type, part or alljof the diluent gas required may beobtained by using a high volatile coal such as -bituminous coal as a reducing agent in the charge of the zinc oxide upon the surface of the charge in-'such furnace; Methane such as present in natural gas may like-` wise be usedas a diluent gas. The slow oxidation of zinc vapor contemplated bythe method of the present invention may be described as combus'e tion of the zinc vapor in a lazy flame under conditions of high temperature maintained fora substantial period of time. zinc 'oxide by the'French process, the zinc `vapor only appears to burn withva lazy ame. How-'vv ever, this slow burning' of zinc vaporv in the In the manufacture of-` French process is merely apparent, since in fact 4 the zinc vapor is being rapidly oxidized on the n surface of a hot innervcone of the flame. Iny consequence, acicular zinc -oxide is not produced in this operation, since-the particles of zincoxide are not permitted to grow in the presence of zinc` Vapor and an oxidizing gas. y

.The crystal habit of the relatively coarse zinc oxides produced in accordance with the method` of the present invention is regulated in accord-Y ancewith'the following principles; f y

When the combustion chamber is maintained' in the lower ranges of the prescribedtemp-erafi tures, say only slightlyiabove-950 C., long thin acicular crystals are obtained. Apparently theser crystals grow in acicular form by successive additions of layers of zinc and-oxygen atomstoth'e basal planes of the-hexagonal zinc'oxide crystals.

45. furnace, or by adding a layer of high volatile coal Y At higher temperatures, the side faces of the crystals tend to grow more rapidly so that thicker columnar needles are formed. In the upper ranges of the prescribed temperatures, growth at the side faces of the crystals is intensied with resultant production of plates and spear-head shaped crystals.

Complex crystal forms such as threelings and the like are obtained under conditions permitting somewhat more rapid rates of growth than is the case with the simpler crystals. High turbulence in the combustion chamber produces more complex and irregular crystals.

The acicular zinc oxide produced in accordance with the present invention is of particular utility as a paint pigment, particularly in paints in tended for outside exposure. Paints containing rrino oxide in coarse particles with highly developed crystal form, in particular acicular crystal form, have less tendency to fail by checking and cracking than paints containing the heretofore customary types of zinc oxide. Moreover, paints made of acicular zinc oxide tend to chalk rather than to crack, that is to say, the paint fails by slow abrasion of the surface instead of b-y the breaking of the paint film into fragments.

The accompanying drawings diagrammatically illustrate apparatus in whichthe improvements of the invention may be practiced.

Fig. l of the drawing illustrates a zinc oxide furnace of the Wetherill grate type as the source of zinc vapor,

Fig. 2 illustrates a volatilization retort as the .source of zinc vapor, and

Fig. 3 illustrates a modified arrangement for introducing Zinc vapor, diluting gas and air into the combustion chamber.

The Zinc oxide furnace of Fig. 1 is of the conventional Wetherill grate type having a perforated fiat grate 5 supporting a charge 6 of zinc ore and reducing material. Air (supplied under pressure through a port 1) is blown through the grate and the charge supported thereon. The furnace is operated as hereinbefore described. The gaseous product of the furnace operation, consisting of zinc vapor and a nonoxidizing diluting gas, is conveyed through a gas outlet 8 inthe roof of the furnace into the combustion chamber 9. Air in controlled amount is supplied slowly to the combustion chamber in any appropriate manner. In Fig. 1 of the drawings,the walls of the combustion chamber are penetrated with a plurality of air inlet ducts I0, which may be equipped with flcwmeters and dampers to facilitate regulation of the amount of air supply. Adequate diluting gas is supplied' by the atmospheric nitrogen traversing the grate 5 of the zinc oxide furnace or by reducing gases evolved from high Volatile coal added to the charge or by both, as hereinbefore described. The supply of air through the ducts I0 in regulated amount and at a slow rate insures the contemplated slow oxidation of the zinc vapor and also avoids excessive turbulence within the combustion chamber. The volume of gases (zinc vapor, diluting gas and oxidizing gas) entering the combustion chamber is so proportioned with respect to the volume of the chamber that the initial particles of zinc oxide are retained in the chamber at approximately their temperature of formation (in excess of 950 C. and preferably exceeding 1000 C.) until most of the particles have grown into relatively coarse crystals of acicular habit. From the combustion chamber the acicular zinc oxide is conveyed in suspension in the accompanying gases through an appropriate flue system to a bag-house or' other zinc oxide collecting device.

In the apparatus of Fig. 2, metallic zinc II is volatilized in a retort I2 electrically heated by electrodes I3. The resulting zinc vapor passes through a gas outlet I4 and mixes with reducing gas sup-plied through a port I5. f The mixed gases pass through an orifice I6 into the combustion chamber I'I. The discharge end `of f the orifice I6 is surrounded by a tubular ringv I8 having an inlet pipe I9 and discharge perforations 2D for introducing air at a slow rate and in regulated amounts into the combustion chamber. The discharge perforations 20 are preferably positioned in the top oi' the ring so that the air streams discharged therethrough are parallel to the stream of Zinc vapor, in order to avoid excessive turbulence and the consequential too rapid oxidation of zinc vapor.

A modified arrangement for introducing' zinc vapor, diluting gas and oxidizing gas (such as air) into the combustion chamber is shown in Fig. 3, Zinc vapor from` a volatilization retort enters the chamberthrough an orifice 2l. The orifice is surrounded by a tubular ring 22 having an inlet pipe 23 and upwardly-directed discharge perforations 2t for introducing the diluting gas into the combustion chamber around the issuing stream of zinc vapor. Air or other oxidizing gas is introduced into the combustion chamber through a tubular ring 25 having an inlet pipe 26 and upwardly-directed dischargeperforations 2?. The zinc vapor is thus surrounded by a curtain of diluting gas (preferably a reducing gas) as it enters the combustion chamber. This ob viates the formation of rock oxide upon the zinc vapor orifice 2l, and at the same time retards the combustion of zinc vapor.

Various other means for supplying air slowly to the combustion chamber may be used in the practice of the invention. Thus, part of the wall of the combustion chamber may be constructed of porous material so as to permit the seepage of air into the chamber. In such a case it may be desirable to provide a screen of streams of diluting neutral or reducing gas in front of they porous portion of the wall and in the `interior of the combustion chamber in order to prevent the deposition of rock oxide on the porous portion of the Wall. l

I claim 1. The improvement in a method of making acicular zinc oxide by the oxidation of zinc vapor which comprises introducing Zinc vapor without substantial preliminary oxidation thereof into a combustion chamber, simultaneously introducing into the combustion chamber a non-oxidizing diluting gas, the volume of the diluting gas being at least twice the volume of the zinc vapor, simultaneously introducing into the combustion chamber air only slightly in excess of that necessary for the oxidation of the zinc vapor, burning the zinc vapor substantially completely with a lazy flame in said chamber in the presence of the air and the non-oxidizing diluting gas, the zinc vapor being introduced into the chamber at such Aa rate that the volume of such vapor entering the chamber per minute does not exceed one twentyfifth of the volume of the chamber.

2. The improvementl in a method of making acicular zinc oxide by the oxidation of zinc vapor which comprises introducing the zinc vapor Without substantial preliminary oxidation thereof and diluted with from 2 to 15 volumes of reducing 1| gas for each volume of zincvapor into a combustion chamber maintained at a temperature `of at least 950 C. and burning the zinc vapor with a lazy llame substantially completely in said chamber in air only slightly in excess of that necessary for the oxidation of the zinc vapor, the zinc Vapor being introduced into the chamber at such a rate that the volume of zinc vapor entering the chamber per minute .ranges from one twentyfth to one ftieth of the volume of the chamber.

r3. The improvement in a method of making acicular zinc oxide by the oxidation of zinc vapor which comprises introducing a jet ofv the zinc vapor without substantial preliminary oxidation thereof into a combustion chamber, simultaneously introducing a jet of loxidizing gas into the combustion chamber at a rate only slightly in excessk of that necessary for the oxidation of the zinc vapor entering the chamber, simultaneously introducing non-oxidizing gas into the chamber and substantially surrounding one of the jets with the non-oxidizing gas, the volume of non-oxidizing gas thus introduced being at least twice the volume of the zinc vapor, burning the zincvapor with a lazy flame substantially completely in said chamber while maintaining the chamber at a temperature in Vexcess of 950 C., the zinc vapor being lintroduced into the chamber at such a rate that the volume of zinc vapor entering `the chamber per minute does not exceed one twenty-iifth ofthe volume of the chamber. 4, The improvement in a method of -rnaking acicular zinc oxide by the oxidation of zinc vapor which comprises introducing the zinc vapor without substantial preliminary oxidation thereof into a combustion chamber maintained at a temperature of at least 950 C., simultaneously introducing into the chamber adjacent a Wall thereof a current of non-oxidizing gas having a `"volumek at least twice that of the zinc vapor, admitting air into the chamber through a porous Wall thereof and admixing the air with the current of non- Y oxidizing gas prior to contact of the air With the zinc vapor, burning the zinc vapor with a'lazy llame substantially completely in saidcharnber, the zinc vapor being introduced intol the chamber at such a rate that the volume of zinc vapor entering the chamber per minute ranges between one twenty-fifth and onefiftiethr of the volume ofV the chamber. l,

5. In the manufacture :of acicular zinc oxide involving the productiony oizincvaporby reactf ing zinc ore with carbonaceous material in a hot bed in which the ore and the carbonaceous mate.- rial are mixed and through Whicha current of' oxidizing and non-oxidizing diluting gas is passed and oxidizing the resultingzinc vapor to produce zinc oxide, the improvement which com-1 prises covering said bed of mixed ore vand car-` bonaceous -material with a layer` of carbonaceous material, bringing the gas rst through the bed and then through the overlying layer into a combustion chamber Without substantial preliminary. oxidation of zinc vapor .accompanying the gas, the volume of the diluting gas accompanying the zinc vapor being at least twice the volume of the* zinc vapor, simultaneously introducing into the combustion chamber air only slightly in excess of that necessary for the oxidation of the Zinc vapori. burning the zinc vapor substantially completely with a lazy flame in suchchamber in the presence of the air and the non-oxidizing diluting gas, the l zinc vapor being introduced into the chamber at j such a rate that the volume cf such vapor entering the chamberv per minute does not exceed one twenty-fifth ofthe volume ofthe chamber. Y HOWARD M. CYR. 

